Primary structures of proteins encoded by genome in various organisms have been revealed based on results of various genome projects. Approximately 30% of proteins in higher organisms have been presumed to be an endogenous membrane protein (integral membrane protein) having transmembrane helix. Membrane proteins are involved in signal transduction, mass transportation, energy production, formation of cytoskeleton etc. at cell membranes. In addition, the membrane protein is very important as a potential drug target. Actually, it is known that approximately 70% of commercially available pharmaceutical agents act on membrane proteins, especially on a G protein-coupled receptor. Protein 3000 Project in Japan which was conducted in consideration of the importance of protein structures has revealed stereostructures of more proteins than was initially expected and produced internationally great results. However, there are merely a few cases that have revealed the stereostructures of membrane proteins, since it is difficult to produce crystals of membrane proteins suitable for structural analysis.
The applicant of the present application has already developed a method for producing an insoluble protein using a cell-free protein synthesis system in the presence of detergent (surfactant) without the protein being insolubilized (see, for example, Patent Document 1 and Non-Patent Document 1). This method can be merely applied to a case where various membrane proteins can be abundantly synthesized as soluble fraction and a case where a detergent used does not inhibit protein synthesis. Accordingly, there are some detergents that cannot be selected for the method as those which contribute to the stabilization of the synthesized protein.
Moreover, a method for synthesizing a membrane protein having a biological function by merely adding a lipid liposome to a cell-free protein synthesis system is also reported (see Patent Document 2 and Non-Patent Document 2). However, it is considered that the method is not necessarily versatile, since the introducibility of membrane proteins into a liposome depends on their respective properties and the probability of contact between a synthesized polypeptide chain and a liposome is low.    [Patent Document 1] Japanese Patent Kokai Publication No. 2003-18999A    [Patent Document 2] Japanese Patent Kokai Publication No. 2005-225796A    [Non-Patent Document 1] Ishihara G. et al., (2005) Protein Expression and Purification, Vol. 41, pp. 27-37    [Non-Patent Document 2] Kalmbach, R. et al., (2007) Journal of Molecular Biology, Vol. 371, pp. 639-648